The present invention relates to a wheeled vehicle having a four wheel drive system capable of varying a driving torque distribution between a front wheel pair and a rear wheel pair, and a four wheel steer system for steering both of the front and rear wheel pairs.
Japanese patent provisional (un-examined) publication No. 58-26634 discloses an example of a conventional four wheel drive vehicle. The four wheel drive vehicle of that publication transmits driving power directly from a transmission to one of the front and rear wheel pairs, and transmits driving power to the other of the front and rear wheel pairs through a transfer clutch. The feature of that four wheel drive vehicle is a switch which is turned on when the automatic transmission is shifted to predetermined ranges with a selector lever, and which is connected in parallel with a manual switch, for selecting either a four wheel drive mode or a two wheel drive mode. In order to fully utilize the four wheel drive system, it is designed to change the drive mode from 2WD to 4WD automatically without need for manual operation when a 2 or 1 range is selected with the select lever for engine braking, starting the vehicle with a heavy load, or driving uphill.
In that four wheel drive system, however, the ratio of an angular displacement of the rear wheels to an angular displacement of the steering wheel is fixed, so that the steering characteristic of the vehicle is changed when the drive mode is changed from 2WD to 4WD and vice versa. In general, front-wheel-drive vehicles tend to understeer, rear-wheel-drive vehicles tend to oversteer, and four-wheel-drive vehicles tend to neutral-steer. Therefore, the driver must change the amount by which he turns the steering wheel in accordance with the drive mode. Vehicles requiring such driving skill are not suitable for average drivers. This problem is serious when the vehicle speed is high.